The present invention relates generally to printed filters and more particularly relates to LC-type printed circuit filters realized using a parallel resonant combination of suspended printed inductor and suspended interdigital capacitor that may be formed over any dielectric substrate.
Wireless radio frequency (RF) communications are becoming more and more prevalent in the world today. Products touting wireless RF communication links are becoming increasingly popular among consumers. A multitude of new products including redesigned existing ones is being built with wireless RF links today. Most RF communication circuits employ some form of resonant circuitry in their transceivers. Due to the explosive consumer demand for products sporting wireless communication links there is a need for low cost, high accuracy and high reliability filters that are suitable for mass manufacture using conventional techniques.
RF filters are necessary circuits in transmitters and receivers that operate in a both wireless and non-wireless, i.e., cable, environments. In a transmitter, the amount of suppression that an RF filter must provide is determined by regulatory requirements or by the amount of interference that the transmitter might cause as a result of unwanted spectral components. RF filters are even more essential in receivers of communications systems especially when the communications system is wireless and is likely to suffer from reception of interfering signals in addition to the natural (thermal) reception noise. In a receiver, the quality of the filtering dramatically effects the reception performance, especially when considering certain types of interference. A particular receiver may deliver an output of degraded quality (i.e., higher error rate in a digital receiver, or severe signal distortions in an analog receiver), if the frequency response of its filters is compromised.
When defining the filtering requirements in a receiver, the following factors should be considered: (1) the frequency band in which the receiver is to operate, (2) the frequency conversions and IF to be used, (3) the spectrum of the modulated signal to be demodulated by the receiver and (4) the nature of any interfering signals to be encountered and the associated rejection requirements.
The filter preceding the demodulator would normally be the narrowest and should allow the minimum amount of additive noise and interference to enter the demodulator. Its bandwidth is normally close to that of the modulated signal for which the receiver was designed. The selectivity of the receiver, i.e., the rejection of adjacent frequencies that may cause jamming or performance degradation, is determined by the steepness of the frequency response curve of the filter.
In a single frequency receiver, such as in the case of simple pagers, the narrow filter may precede most of the electronic circuitry of the receiver, thus reducing the possibility of intermodulation produced within the receiver. In receivers intended for an entire band of frequencies, from which one frequency among many is in use at any one time, the filtering at the input of the receiver usually has a bandwidth at least as wide as that of the entire band used. These filters cannot provide rejection for in band interference and usually do not have significant attenuation even for out of band frequencies that are close to the edges of the band. In such receivers, the IF filtering provides effective rejection of such interference as long as an inband intermodulation product was not generated before the signal reaches the narrow IF filtering.
The wide filters located at the input to the receiver are intended to provide image rejection and out of band interference rejection, which is effective for signals sufficiently far from the edges of the frequency band. A typical filter, e.g., surface acoustic wave (SAW) filter, for use in the 900 MHz ISM band, for example, costs over $1.00 and has a frequency response with limited out of band attenuation.
Most RF filters and circuits for communication applications make use of one or more inductors in their design. Previously, these were lumped inductors or printed inductors that have been formed on printed circuit boards using a variety of techniques such as stripline, microstrip, slotline, etc. Inductors formed using any of these techniques are typically constructed in the form of a planar spiral with the spiral being circular or square in shape. A disadvantage, however, of forming inductors, such as microstrip inductors, on printed circuit boards is that they are very sensitive to the characteristics of the printed circuit board material. The characteristics of the printed circuit board material directly affect the characteristics and performance of the inductors formed thereon. Parameters of the PCB material such as thickness and dielectric constant affect the characteristics of the inductor. The sensitivity of microstrip inductors to the dielectric constant of the printed circuit board material results in variations in the resultant self-resonant frequency. In addition, variations in the thickness of the PCB material causes variations in the value of the inductance that results in frequency response errors of any filter constructed therefrom.
Another disadvantage of constructing printed inductors on printed circuit boards using traditional techniques is that the repeatability of the value of the inductance is too low for mass production. As described above, the characteristics of the inductor are very sensitive to the parameters of the printed circuit board material. In addition, most printed circuit inductors constructed utilizing conventional techniques have limited values of the quality factor Q of the inductor. This is due to the nature of the conventional inductor that is constructed having a ground plane. Further, since the ground plane is separated from the printed inductor traces by the printed circuit board material there is typically significant parasitic capacitance between the inductor and the underlying ground plane. In some applications, this parasitic capacitance can be problematic because it causes a reduction in the self-resonant frequency of any LC combination formed using the inductor.
An alternative to using printed circuit inductors, such as microstrip and stripline inductors, is to use discrete inductor elements. A disadvantage, however, of using discrete elements is the high cost typically associated with high Q lumped coils.
A limitation, however, of the use of filters that utilize printed inductors is the sensitivity of the center frequency and other filter characteristics to variations in the PCB etching process. One means of compensation is to design wider filters to accommodate the PCB etching tolerances. This, however, is not always a viable solution, as the requirements of the particular application may demand sharp narrow filters.
Another prior art solution to realizing filters with invariable center frequencies is to use precise PCB etching techniques. High precision PCB manufacturing, however, entails higher manufacturing costs.
The effect of underetching of the traces results in a lower inductance value due to less magnetic flux linkage since there is less space for the magnetic field in between the traces. Conversely, overetching results in a higher inductance for the opposite reason. For example, a variation in trace width (e.g., 8 mil rather than the desired 10 mil) of 2 mil may result in a center frequency deviation of 2% (or a 20 MHz error in the desired center frequency of a 900 MHz band filter). In practice, variations in PCB etching tolerances may result in a filter having a variation in center frequency of 3%. Furthermore, the bandwidth of such a filter may be extended from 28 MHz to 45 MHz. In the ISM bands allowed by the FCC for unlicensed operation, for example, such a filter is too wide to sufficiently filter out unwanted signals from outside the desired band.
The present invention is a suspended printed inductor (SPI) connected in parallel to a suspended interdigital capacitor (SIC) so as to form a parallel resonant circuit that is nearly independent of variations in PCB etching tolerances. This combination of SPI and SIC functions to resonate at a center frequency, and with similar parallel resonant circuits can be used to form RF filters having any desired order. Using the parallel resonant combination of SPI and SIC, an RF filter can be constructed whose electrical properties are nearly insensitive to variations in PCB parameters and etch processing. The sensitivity of the spiral suspended printed inductor, in combination with the suspended printed interdigital capacitor, to PCB parameters such as dielectric constant and PCB height, is greatly reduced. Further, the parallel combination of suspended printed spiral inductor and suspended interdigital capacitor is nearly insensitive to PCB etching tolerances.
The inductance of the SPI is inversely proportional to its trace width. The wider the trace (which may be caused by underetching), the lower the magnetic flux surrounding the trace and hence, the lower the inductance. The lower the inductance, the higher the resonant frequency. Conversely, the narrower the trace (which may be caused by overetching), the higher the magnetic flux surrounding the trace and hence, the higher the inductance. The higher the inductance, the lower the resonant frequency.
The capacitance of the SIC, on the other hand, is directly proportional to its trace width. The wider the trace, the smaller the distances between the fingers of the capacitor and hence, the higher the capacitance. The higher the capacitance, the lower the resonant frequency. Conversely, the narrower the trace, the larger the distances between the fingers of the capacitor and hence, the lower the capacitance. The lower the capacitance, the higher the resonant frequency.
Thus, the effects of over and underetching on the inductance of the SPI are inversely related to those of the capacitance of the SIC. The effects on the SPI and SIC can be combined in a parallel fashion such that they complement and annul each other. The parallel combination of SPI and SIC can be constructed such that the resonant frequency is nearly independent of the trace width. Thus, the center frequency and the bandwidth of an RF filter (e.g., low pass, high pass, band pass or band stop) constructed therefrom can be adapted, utilizing suitable parallel resonant circuits comprised of SPI and SIC components, to be nearly independent of variations in the PCB etching process.
The basic element of the printed filter of the present invention is the suspended printed inductor (SPI) and suspended interdigital capacitor (SIC). A characteristic feature of a SPI and SIC is that their ground planes are effectively removed. A metal shield connected to ground may be used, e.g., a metal shield surrounding the circuit board, but it is placed at a sufficient distance form the printed circuit board such that the distance can be considered virtual infinity from an RF perspective.
The parallel resonant combination of SPI and SIC can be utilized to construct numerous types of filters such as low pass, high pass, band pass, band stop or any combination thereof. In each case, the filter can be used in lumped or semi-lumped networks together with different types of capacitors such as SMD ceramic, thin film printed or interdigital printed capacitors.
The parallel resonant combination of SPI and SIC has many advantages which are briefly outlined below. First, the use of the SPI with a parallel SIC functions to greatly reduce the effects of variations in the trace widths caused by variations in the PCB etching process. As described hereinabove, the effect of variations in trace widths may result in unacceptable variation in filter parameters, e.g., center frequency, bandwidth, etc.
Second, a suspended printed inductor has a high quality factor Q even when constructed using low cost printed circuit board (PCB) materials. A quality factor Q can be achieved that is up to ten times that of a microstrip inductor implemented on the same material.
Third, suspended printed inductors have improved repeatability of the inductance value. Since there is no ground plane under the printed inductor, sensitivities to PCB material characteristics such as thickness and dielectric constant, are eliminated. The variation of the inductance value of an SPI, i.e., the tolerance, due to the ground plane distance can be reduced to less than 1%, which helps to eliminate the need for trimming or tuning during production.
Fourth, the self-resonance frequency of a suspended printed inductor is typically much higher than that of a microstrip inductor. This is due to the low parasitic capacitance of suspended printed inductors. A low parasitic capacitance is achieved by increasing the distance to the ground plane or by removing it altogether, which functions to reduce the effective dielectric constant value of the inductor. This permits higher inductance values to be achieved while maintaining a safe frequency distance from the self-resonant frequency of the inductor.
Fifth, suspended printed inductors are cheaper to manufacture when compared to the use of lumped elements. SPIs serve to eliminate the high costs of high Q lumped coils by replacing them with printed patterns of negligible cost on a printed circuit board.
There is thus provided in accordance with the present invention a suspended printed resonant circuit comprising a substrate onto which circuit elements can be printed, a first electrically conductive trace printed onto the substrate, the first electrically conductive trace shaped so as to form an inductor at the high radio frequencies of interest, a second electrically conductive trace printed onto the substrate, the second electrically conductive trace shaped so as to form an interdigital capacitor at the high radio frequencies of interest, the interdigital capacitor electrically connected in parallel to the inductor so as to form a parallel resonant circuit, the resonant frequency of the parallel resonant circuit being substantially independent of variations in the width of the first electrically conductive trace and the second electrically conductive trace and wherein an area of the substrate underlying the first electrically conductive trace and the second electrically conductive trace is devoid of electrical traces, including power, ground and signal traces.
There is also provided in accordance with the present invention a radio frequency (RF) filter having an input and an output, the filter for filtering an input signal to yield an output signal therefrom, the filter comprising a substrate onto which circuit elements can be printed, a filter circuit comprising N poles and N zeros wherein each pole and each zero comprises a parallel resonant circuit comprising a suspended printed inductor in parallel with a suspended interdigital capacitor, wherein the resonant frequency of the parallel resonant circuit being substantially independent of variations in the width of the traces used to construct the suspended printed inductor and the suspended interdigital capacitor, coupling means for electrically coupling the N poles and the N zeros to one another in daisy chain fashion, input coupling means for coupling the input signal to the filter circuit, output coupling means for coupling the filter circuit to the output thus forming the output signal and wherein N is a positive integer greater than or equal to one.
There is further provided in accordance with the present invention a method of fabricating a suspended printed resonant circuit, the method comprising the steps of providing a substrate onto which circuit elements can be printed, printing onto the substrate, a first electrically conductive trace using standard lithographic techniques such that the first electrically conductive trace functions as an inductor at the RF frequencies of interest and printing onto the substrate, a second electrically conductive trace using standard lithographic techniques such that the second electrically conductive trace functions as an interdigital capacitor at the RF frequencies of interest and such that the interdigital capacitor is electrically connected in parallel to the inductor so as to form a parallel resonant circuit and creating an area on the substrate substantially underlying the first electrically conductive trace and the second electrically conductive trace that is devoid of electrical traces, including power, ground and signal traces.